1. Field of the Invention
The present invention relates to electric motors and, in particular, relates to an electric motor having a rotor-embedded sensor. In a particularly preferred embodiment, the present invention relates to an induction motor having a rotor-embedded sensor that measures the current through one or more rotor windings.
2. Description of Related Art
Electric motors comprise a stator and a rotor that is capable of rotain relative to the stator. For precise motor control, it is generaly desirable to know as much as possible regarding motor operating conditions, including conditions that are parameters which can be measured and assigned a value. Consequently, a wide variety of sensors have been provided that are usefuil for sensing motor operating conditions. For example, position and velocity tranducers have been provided that can be mounted to an output shaft of a motor so that the angular position and/or angular velocity of the motor may be measured. As well, current sensors have been provided that can be coupled to the stator windings so that the current flowing the stator windings may be measured.
However, few if any sensors have been provided that are capable of directly measuring conditions associated with the rotor. For example, no known sensor has been provided that is capable of measuring current in the rotor windings of an induction motor. In an induction motor, current is induced in the rotor windings magnetically and, as a result, there are no electrical connections between the rotor windings and the remainder of the motor that can serve as the basis for performing such measurements. Thus, the rotor current cannot be directly measured using existing approaches.
Nevertheless, the ability to perform such measurements would be extremely useful. The magnitude of the current in the rotor windings of an induction motor is indicative of the torque produced by the motor shaft. Additionally, the frequency of the current in the rotor windings is the slip frequency, which can be used to determine the speed of the motor shaft. Finally, knowledge of the phase of the rotor current could be used to synchronize the phase of the current that is applied to the stator windings with the angular position of the rotor. Thus, the ability to measure rotor current would be extremely useful because it would enhance the ability to precisely control the induction motor.
Beyond motor control, the ability to measure rotor current would also be extremely useful for other reasons. For example, the rotor windings of an induction motor are generally formed of solid metal bars that are subject to fatigue and breaking. Currently, broken rotor bars are diagnosed by the acoustical noise and/or the excessive heat that is generated when a rotor bar breaks. However, it is often not possible to diagnose breaking in this way until several rotor bars windings are affected, by which time the motor must be replaced immediately. Therefore, it would be extremely useful to be able to measure the rotor current as a way of detecting broken rotor bars.
Other parameters that are useful to measure include rotor temperature, rotor airgap, rotor flux and rotor torque. Measuring these parameters is useful not only in conjunction with induction motors, but also in conjunction with all types of motors.
Accordingly, what is needed is a way to measure these and other rotor-associated operating conditions.
The present invention provides a way to measure rotor-associated operating conditions, such as rotor current (including the magnitude, frequency and phase of the current), rotor fatigue, rotor temperature, rotor airgap, rotor flux and rotor torque. In general, the present invention may be used with all types of electromechanical devices including both electric motors and electric generators. In an especially preferred embodiment, the present invention provides a way to measure the rotor current in an induction motor.
According to one aspect of the invention, the present invention provides an electromechanical device that comprises a stator and a rotor which is capable of rotating relative to the stator. The rotor has a sensor embedded therein. In a preferred embodiment, the sensor is disposed within a plurality of laminations that are stacked one on top of another. For example, one of the plurality of laminations may be a sensor lamination, in which case the sensor is at least partially disposed within a cavity formed in the sensor lamination.
According to a preferred aspect of the invention, the sensor is a current sensor. For example, the current sensor may comprise a current transformer having a primary winding that is partially formed by a rotor bar that forms part of a rotor winding. Alternatively, a current sensor may be used that has a magnetic core which substantially encircles one of the rotor bars and a Hall effect device which is interposed between first and second adjacent ends of the magnetic core. The current measurement (magnitude, frequency, and/or phase) that is thereby obtained may then be used as a basis for controlling the motor. Alternatively, the sensor may also comprise a plurality of current sensors, each which measures current through a respective rotor bar. In this case, the sensor could be used to sense rotor bar fatigue.
According to another aspect of the invention, the present invention provides a method comprising providing an electric motor having a stator and a rotor that rotates relative to the stator, and sensing a motor operating condition using a sensor. The rotor includes the sensor that is used to sense the motor operating condition. Accordingly, the method also comprises the steps of transmitting information pertaining to the motor operating condition from the rotor to a motor control system. The motor control system is used to control the motor at least partially based on the information pertaining to the motor operating condition. Preferably, during the sensing step, the sensor senses current flowing through a rotor bar that partially forms a rotor winding of the rotor. In this case, the motor control system controls the motor based at least partially on the current measurement that is provided by the current sensor.
Other objects, features, and advantages of the present invention will become apparent to those skilled in the art from the following detailed description and accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many modifications and changes within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.